3 Other Positioning SystemsRussian SYSTEM Know as GLONASSAround 9 Operational SatellitesHasn’t Reached Full Operational CapabilityEuropean System Known as GALILEO2 Satellites launched yet inSystem Will Start To Operate 2013.All Systems Will Combine To Form GNSS, Global Navigation Satellite SystemsAtlas Gis

4 Global Positioning System GPSGlobal Positioning System GPS Developed by the US Dep. of Defense in 1974( Civil Access In 1995 )Navigational GPS Up to m AccuracyDeferential GPS Up to 0.1 mm AccuracyAtlas Gis

5 General CharacteristicsNavigation System with Time and Ranging -Global Positioning SystemDesigned to replace existing navigation systemsDeveloped by the US Dep. of DefenseDevelopment started in 1974According to military aspectsWorldwide Coverage24 hour accessAccurate Navigation to 10 mAtlas Gis

12 GPS Principle : Point Positioning3 Spheres intersect at a point3 Ranges to resolve for Latitude, Longitude and HeightR32 Spheres intersect as a circleR2We are somewhere on a sphere of radius, R1Atlas Gis

13 Outline Principle : PositionThe satellites are like “Orbiting Control Stations”Ranges (distances) are measured to each satellites using time dependent codesRuntime: Time from the transmission of the signal at the satellite till the reception at the receiverMultiply the runtime with the velocity of lightRange = Runtime x velocity of lightNotes :

14 Runtime Determination (Code)One-way ranging system: One clock inside the satellite and another one inside the receiverReceiver creates a duplicate of the codeCross correlation between received and generated code sequenceMaximum correlation gives the runtimetSatellite SignalReceiver SignalAtlas Gis

16 Atlas Gis Pseudorange The PseudorangeTypically GPS receivers use inexpensive clocks. They are much less accurate than the clocks on board the satellitesRuntime measurement is done in the receiverReceiver time scale has an offsetConsider an error in the receiver clock1/10 second error = 30,000 Km error1/1,000,000 second error = 300 m errorA radio wave travels at the speed of lightactual signal velocity differs from theoretical valueInstead of the true distance Satellite - Receiver we obtain thePseudorangeAtlas Gis

17 Atlas Gis Point Positioning4 Ranges to resolve for Latitude, Longitude, Height & TimeIt is similar in principle to a resection problemAtlas GisNotes :

18 Range Determination from Phase ObservationPhase ObservationsWavelength of the signal is 19 cm on L1 and 24 cm on L2Receiver compares self-generated phase with received phaseNumber of wavelengths is not known at the time the receiver is switched on (carrier phase ambiguity)As long as you track the satellite, the change in distance can be observed (the carrier phase ambiguity remains constant)Received SatellitePhaseGeneratedPhase fromReceiverDTD = c DT + lNAtlas GisNotes :

19 Atlas Gis Error SourcesLike all other Surveying Equipment GPS works in the Real WorldThat means it owns a set of unique errorsAtlas Gis

20 Atlas Gis Satellite Errors Satellite Clock Modelthough they use atomic clocks, they are still subject to small inaccuracies in their time keepingThese inaccuracies will translate into positional errors.Orbit UncertaintyThe satellites position in space is also important as it’s the beginning for all calculationsThey drift slightly from their predicted orbitAtlas Gis

21 Atlas Gis Observation ErrorsGPS signals transmit their timing information via radio wavesIt is assumed that a radio wave travels at the speed of light.GPS signals must travel through a number of layers making up the atmosphere.As they travel through these layers the signal gets delayedThis delay translates into an error in the calculation of the distance between the satellite and the receiver19950 KmIonosphere200 KmTroposphere50 KmAtlas Gis

22 Atlas Gis Receiver ErrorUnfortunately not all the receivers are perfect. They can introduce errors of their ownInternal receiver noiseReceiver clock driftAtlas Gis

23 Atlas Gis Multipath ErrorWhen the GPS signal arrives at earth it may reflect off various obstructionsFirst the antenna receives the signal by the direct route and then the reflected signal arrives a little laterAtlas Gis

24 Point Positioning AccuracyAccuracy mIn theory a point position can be accurate to m based on the C/A Code( Navigational)Atlas Gis

26 Point Positioning AccuracyAs the owner of the GPS System the USA have decided to limit theaccess to the full system accuracy for civil usersSPS Standard Positioning ServiceBased on the C/A-CodeWorldwide available without limitationNavigation Accuracy von ±100 m in position and ±160 m in height (~95 % probability)PPS Precise Positioning ServiceProvides the full System accuracyC/A-Code gives ±10-30 m, P-Code ±20 m (~95 %)PPS is available for authorized users onlyAtlas Gis

29 Differential Code / PhaseIf using Code only accuracy is in the range of cm This is typically referred to as DGPSIf using Phase or Code & Phase accuracy is in the order of 3 mm + 0.5Baseline VectorBAAtlas Gis

30 Initial Phase AmbiguityInitial phase Ambiguity must be determined to use carrier phase data as distance measurements over timeOnce the ambiguities are resolved, the accuracy of the measurement does not significantly improve with timeAtlas GisNotes :

32 Dilution of Precision (DOP)A description of purely geometrical contribution to the uncertainty in a position fixIt is an indicator as to the geometrical strength of the satellites being tracked at the time of measurementGDOP (Geometrical), Includes Lat, Lon, Height & TimePDOP (Positional) Includes Lat, Lon & HeightHDOP (Horizontal) Includes Lat & LonVDOP (Vertical) Includes Height onlyPoor GDOPGood GDOPAtlas Gis